The present invention relates, in general, to gas analyzers for analyzing the presence and/or quantity of a gas, and in particular to a new and useful arrangement and method of calibrating gas analyzers using a test gas of known properties.
Gas analyzers are known which use aspirators that function to draw a sample gas from an environment, for example from the interior of a gas duct, and bring that sample to a sensor or other sampling system which performs an analytical test on the sample.
The sample gas is drawn by a probe to the sensor or sampling system. Such probes usually extend into the environment to be tested and are used in conjunction with a filter for filtering out debris.
Such gas analyzers are known to require some mechanism for applying a test gas in order to accurately check the calibration of the instrument. Simply flooding the system with the test gas to overcome the sample gas entering the probe is not an accurate check since this pressurizes the system which is normally operated at negative pressure.
Such a method also cannot check for the existence of leaks in the normally negatively pressurized system.
Such a method also produces more flow rate through the sensor cavities which will represent an error particularly in certain types of sensors which are sensitive to flow rate. This would also represent an error in sensors which utilize a test gas that is diluted with air at constant flow, for example in the case of carbon monoxide sensors which require dilution air for combustion of the catalyst.
Other methods of calibrating a gas analyzer include physically blocking the probe to prevent entry of the sample gas and to prevent mixing between the sample gas and the test gas. This is usually accomplished by physically inserting a plug into the probe or into a connecting passage for the probe. The test gas is then injected into a port in the sampling or analyzing system which is downstream of the mechanical plug. Shortcomings of this method include frequent failures of seals used to block off the same probe or failures in seals associated with shafts used for moving the plug assembly for plugging the probe. Other problems include seizing between two relatively movable parts where metal to metal thread seals are utilized. This is due to the high temperatures required by some sampling systems, as well as high temperatures which may exist in the duct or environment containing the sample gas. In addition to the foregoing problems, the prior art methods require a local calibration unless a solenoid shut-off is used which is also effected by high temperatures and require seals.